1. Field of the Invention
The present invention relates to a piezoelectric device including a piezoelectric single crystal thin film, and more specifically a piezoelectric device including a membrane structure, and a method for manufacturing the piezoelectric device.
2. Description of the Related Art
Currently, a large number of piezoelectric devices in which a piezoelectric single crystal body is formed in a thin film have been developed. In the piezoelectric devices containing such a piezoelectric thin film, a support member for supporting the piezoelectric thin film is required in practical use. Such a support member is disposed on one principal surface of the piezoelectric thin film as described in Japanese Unexamined Patent Application Publication No. 2007-228319 or Japanese Unexamined Patent Application Publication No. 2003-17967.
Some of the piezoelectric devices have a membrane structure in which, a space is formed between a region in which an electrode which functions as the piezoelectric device in the piezoelectric thin film is formed and the support member, in order to increase the characteristics of the devices.
Heretofore, a smart cut method has been used as one of the methods for forming a composite piezoelectric substrate containing a piezoelectric thin film and a support member. According to the smart cut method, an ion implanted layer is formed by implanting ions into one principal surface of a piezoelectric substrate having a joinable thickness. Next, the support member which is separately formed is joined to the principal surface at the ion implanted layer side of the piezoelectric substrate on which the ion implanted layer is formed by using an activation junction, affinity junction, or the like. Thereafter, the piezoelectric thin film is separated by heating from the piezoelectric substrate utilizing the ion implanted layer.
Therefore, when forming the piezoelectric device having the membrane structure, a sacrificial layer which serves as a gap layer later is formed on one surface of the piezoelectric substrate, and then the support member is joined to the front surface of the piezoelectric substrate on which the sacrificial layer is formed. Thereafter, the piezoelectric thin film is separated from the piezoelectric substrate to be formed, etching windows are formed in the piezoelectric thin film, and then the sacrificial layer is removed from the etching windows. In this case, a structure in which the piezoelectric thin film and the support member are joined to each other at portions other than the membrane structure portion is formed.
Mentioned as a method of joining the substrate for separating and forming a thin film and the support member is a method including placing an elastic body between a single crystal silicon substrate, which is not a composite piezoelectric substrate, and a support as described in Japanese Unexamined Patent Application Publication No. 2008-118079.
However, when directly joining a piezoelectric substrate and a support member as described in Japanese Unexamined Patent Application Publication No. 2007-228319 or Japanese Unexamined Patent Application Publication No. 2003-17967, the coefficient of linear expansion difference between the piezoelectric substrate and the support member cannot be disregarded, and therefore materials of the support member are considerably limited. Moreover, in order not to apply unnecessary stress to the piezoelectric substrate during the junction process, it is required to set strict joining conditions in such a manner that irregularities exceeding a certain level are not present or particles (foreign substances, dust, or the like of a minimum size) are not present on a joined surface, which increases the process load and makes the process control difficult.
In particular, when forming the membrane structure, not only the flatness degree of the piezoelectric substrate surface but the flatness degree of the sacrificial layer surface needs to increase. The sacrificial layer surface and the piezoelectric substrate surface cannot form the same plane in terms of a manufacturing process. Therefore, in order to increase the flatness degree thereof to a certain level or higher, the process load increases, which leads to an increase in cost.
In contrast, as described in Japanese Unexamined Patent Application Publication No. 2008-118079, various problems occurring when joining the piezoelectric substrate and the support member as described above are lessened by inserting an elastic body between the semiconductor substrate and the support member. However, by joining an elastic body with a low elastic coefficient to the piezoelectric substrate, damping occurs, so that the function as the piezoelectric device decreases. This phenomenon arises without exception even in the case of the membrane structure because at least a portion where the piezoelectric thin film and the elastic body are joined to each other is present. Particularly in a piezoelectric device utilizing elastic waves, the influence is considerable.